Ground proximity warning system utilizing radio and barometric altimeter combination

ABSTRACT

A ground proximity warning system for an aircraft which produces a warning signal when the rate of descent of the aircraft exceeds a limiting value determined by the aircraft altitude. The rate of descent of the aircraft is calculated from a combination of measurements of the altitude of the aircraft above ground and the barometric altitude of the aircraft. A signal representing the rate of change of the altitude of the aircraft above ground is limited, to minimize false warnings as a result of surface irregularities. The barometric portion of the system is disabled during take-off run and the initial portion of the aircraft climb-out to avoid a false warning resulting from an increased barometric pressure condition during this maneuver. The limits on the aircraft altitude rate signal are modified in accordance with the flight mode of the aircraft, to reduce the sensitivity of the warning system during the final stage of a landing approach. The warning system provides an alarm to the pilot which is an audio tone modulated at a low frequency repetition rate. A soft warning is actuated when the sink rate of the aircraft exceeds a desired level for the altitude at which the aircraft is operating. If the aircraft is also below a limiting altitude, a hard warn signal is provided. The repetition rate of the warning signal, while in a soft warn condition, is a function of the difference between the altitude of the aircraft and the altitude at which the sink rate would be proper. A complementary filter with which the altitude rate signals are combined, includes an operational amplifier connected in an unloading amplifier configuration with a resistive circuit connecting one input signal with the amplifier and a capacitive circuit connecting the other input signal with the amplifier, the resistive and capacitive input circuits forming a low pass filter for the first signal and a high pass filter for the second signal.

finite States Patent Astengo Feb. 6, 1973 [54] GROUND PROXHMITY WARNINGSYSTEM UTTLHZHNG RADIO AND BAROMETRIC ALTHMETER COMBINATION [75]Inventor:

descent of the aircraft exceeds alimiting value determined by theaircraft altitude. The rate of descent of the aircraft is calculatedfrom a combination of measurements of the altitude of the aircraft aboveground and the barometric altitude of the aircraft. A signalrepresenting the rate of change of the altitude of the aircraft aboveground is limited, to minimize false warnings as a result of surfaceirregularities. The barometric portion of the system is disabled duringtake-off run and the initial portion of the aircraft climb-out to avoida false warning resulting from an increased barometric pressurecondition during this [52] US. Cl. ..340/27 R, 340/213 R, 330/31maneuveh The limits on the i ft altitude rate [51] lint. Cl. ..Glc /00Signal are modified in accordance with the flight mode [581 Flew Search"340/27 37 NAI 12 of the aircraft, to reduce the sensitivity of thewarning 340/1 12 A system during the final stage of a landing approach.

[56 References Cited The warning system provides an alarm to the pilotwhich is an audio tone modulated at a low frequency UNlTED STATESPATENTS repetition rate. A soft warning is actuated when the 3 093 8076/1963 Crane ..343/l2 A Sink rate the aircraft exceeds a desired levelfor i 2:931:22} 5/1960 Rusk altitude at which the aircraft is operating.If the an- 3 140 433 7 9 g M343/H2A craft is also below a limitingaltitude, a hard warn 3,077 557 2 19 3 joline 340 27 R signal isprovided. The repetition rate of the warning 2,735,081 2/1956 Hosford..340/27 NA signal, while in a soft warn condition, is a function ofOTHER PUBLICATIONS Philbrick/Nexus Research, Applications Manual forOperational Amplifiers, P. 46, 1968.

Primary Examiner-KathleenH. Claffy Assistant Examiner-Jon BradfordLeaheey Attorney-Hofgren, Wegner, Allen, Stellman & Mc- Cord ABSTRACT Aground proximity warning system for an aircraft which produces a warningsignal when the rate of the difference between the altitude of theaircraft and the altitude at which the sink rate would be roper. Acomplementary filter with whIch the al Itude rate 17 Claims, 9 DrawingFigures 40 COMPLEMENTARY FILTER I TRIP EQUATION COMPUTER low PASS FILTERc 1 RADIO R cl 2 [53 I ALTIMETER I p-J AIRCRAFT cw CQNFIGURATION SENSOR55 l BAROMETRIC c ALTIMETER (1' 8+!) 3 TD 54 I 4/ 4a 1 J 7 h eIARomErRIcB HIGH PASS I 1 W ALT M FILTER I I f //6 ll? //9 1 I03 I 1 1 SW m m? I/06 -H I I I FREQ MODU w f2 can. unoa NORMAL A I D AND ACCELERO- N gn gI I I METER I I I /12 i I I a -4 I RIZP I I I RATE I. a 1 L a l AuoIoLOGIC AUDIO GENERATOR PMENIEDFEB 6 I975 3.715.718

SHEET 10$ 3 WARNING INITIATION OF v PULL UP 0 E Z I! 7 3 SINK RATE ATWARNING ALTITUDE, h'

TRIP EQUATION COMPUTER F 1 3 20 2/ MULTIPLIER 24 j 2 I w ALTITUDE h s hI h l SENSOR Zhg I COMPARZATOR II-J 23 25 [PT 9 w I 25 I CLOSURE 2 IWARNING SIGNAL I T I )g mvsmon L fifl'azezgo Z -f7*- f w Vera... r w

ATTORNEYQ PATENTED FEB 6 I973 3,715 718 SHEET 3 OF 3 mOP muZw0 05. 200'.053a GROUND PROXIMITY WARNING SYSTEM UTILIZING RADIO AND BAROMETRICALTIMIE'IER COMBINATION This invention is concerned with an instrumentfor indicating to the pilot a condition of proximity to the ground whichrequires correction; i.e., the movement of an aircraft into arelationship with the ground which might result in a crash unless thepilot executes a climb. It has been proposed that such a warning begenerated in accordance with a combination of aircraft altitude andaltitude rate. It will be recognized, for example, that an aircraft atan altitude of a thousand feet above the ground can descend safely at amore rapid rate than an aircraft at one hundred feet. Systems which havebeen proposed have been unsuccessful primarily as a result of the lackof a satisfactory measure of aircraft rate of change of altitude.

Instruments are available to measure the altitude of an aircraft abovethe ground, as radio altimeters, and the output of a radio altimeter canbe differentiated to obtain a signal representing the rate of change ofaircraft altitude with respect to the ground. Such a signal issatisfactory as long as the ground surface is relatively smooth. Where,however, the ground is irregular or the aircraft flies over largebuildings, trees or the like, altitude rate signals are generated whichcause false warnings. It is also known to measure the altitude of anaircraft barometrically with respect to a fixed reference, as sea level.The output of a barometric instrument may be differentiated to obtain asignal representing the barometric rate of change of altitude of theaircraft. Such a signal is satisfactory in a ground proximity warningsystem as long as the ground is relatively flat. However, if the groundover which the aircraft operates is rising, for example, no warning willbe given even though the aircraft flies into the ground.

Attempts to filter the signal representing rate of change of altitudewith respect to the ground, to remove the high frequency components,have been unsuccessful in reducing the problem of false warnings. if thefilter time constant is large enough to eliminate the high amplitudesignals from ground irregularities, the system responds so slowly tochanges in the aircraft altitude that it is unsatisfactory.

l have found that the false warnings can be significantly reduced, ifnot completely eliminated, without impairing the sensitivity of thesystem, by limiting the amplitude of the signal representing the rate ofchange of altitude with respect to the ground.

One feature of the invention is the provision of means for generating asignal representing the aircraft altitude rate of change, for use in aninstrument for determining the proximity condition of the aircraft withrespect to ground, including means for measuring the altitude of theaircraft with respect to the ground, means responsive to the output ofthe altitude measuring means for deriving a signal representing the timerate of change of aircraft altitude and means for limiting the amplitudeof the aircraft altitude rate signal, reducing sensitivity of theinstrument to irregularities of the ground surface.

Another feature of the invention is that the amplitude limit of thealtitude rate signal is adjusted as a function of the flight mode of theaircraft. For example, if a cruising aircraft unintentionally approachesthe ground, it is desirable that a warning be given as soon as thisrelation to the ground reaches a predetermined condition from which itcan readily climb to a safer altitude. However, when the aircraft is ona landing approach, intentionally approaching the ground, the amplitudeof the altitude rate signal is limited to a lower level to avoidunnecessary false warnings.

A further feature of the invention is that the limited altitude ratesignal is combined with a signal representing the rate of change of thebarometric aircraft altitude, in a complementary filter which selectsthe low frequency or long term components of the limited rate signal andthe short term, high frequency components of the barometric rate signal.The calculated altitude rate signal from the filter has a high degree ofaccuracy while eliminating inaccuracies due to ground irregularity andfrom offsets and drift of the barometric altimeter.

Yet another feature of the invention is the provision in a groundproximity instrument utilizing a means responsive to air pressure formeasuring barometric altitude of the aircraft of a means for disablingthe altitude measuring means during take-off and the initial portion ofthe climb-out. During the take-off run of an aircraft the air around ittends to compress as it is forced between the plane and the surface ofthe runway. This results in a low reading on a barometric altimeter anda rate signal which shows that the plane is descending although it isstill on the ground. Accordingly, the barometric altimeter cut-outavoids the occurrence of a false warning.

And a further feature of the invention is the utilization of a signalfrom a normal accelerometer either in place of or to complement thebarometric altitude rate signal, providing dynamic altitude rateinformation for combination with the signal representing the time rateof change of measured altitude with respect to ground.

Another feature of the invention is the provision of a system forwarning a machine operator of an undesired operating condition includingmeans establishing first and second warning criteria and meansresponsive to each criteria for establishing first and second alarms. Ifthe operator does not react to the first warning, the occurrence of thesecond warning should demand a response.

Still a further feature of the invention is the provision of means formeasuring the difference between the operating condition of the machineand a boundary between desired and undesired operating condition. Acharacteristic of the alarm is varied as a function of this difference.More particularly, an audible alarm is modulated at a repetition ratewhich is varied as a function of the difference between the operatingcondition and the boundary.

And another feature of the invention is the provision of a complementaryfilter utilizing an operational amplifier having a feedback circuitbetween the output and the negative input, for operation of theamplifier as an unloading amplifier. A resistive circuit connects onesignal source with the positive input while a capacitive input connectsa second signal source with the positive amplifier input. The resistiveand capacitive circuits form a low pass filter for the signal from thefirst source and a high pass filter for the signal from the secondsource. The time constant of both filters may be varied by simplyadjusting the resistance of the resistive circuit.

Further features and advantages of the invention will readily beapparent from the following specification and from the drawings, inwhich:

FIG. 1 is a curve representing the trajectory of an aircraft in asituation of recovery from a ground proximity warning;

FIG. 2 is a plot of warning altitude as a function of the sink rate ofthe aircraft for a representative ground proximity warning system;

FIG. 3 is a functional block diagram of a warning system based on thecurve of FIG. 2;

FIG. 4 is a functional block diagram of a system embodying the inventionfor computing an altitude rate signal for the warning system of FIG. 3;

FIGS. 40, 4b and 4c illustrate the signal-amplitudefrequencycharacteristics of the complementary filter of FIG. 4.

FIG. 5 is a functional block diagram of a preferred form of the groundproximity warning system embodying the invention; and

FIG. 6 is a schematic diagram of a circuit for the altitude rate limiterand the complementary filter.

The ground proximity warning system disclosed herein is based oncriteria that have been proposed by others, but which have notpreviously been successfully utilized. Briefly, the warning criteria areaircraft al titude with respect to the ground and the rate of change ofaltitude. When these two factors have a certain relation, as willappear, the pilot is given a warning that he should execute a pull-upmaneuver, causing the aircraft to climb. The principal problem inutilizing such a warning has been an inability to generate asatisfactory signal representing the rate of change of aircraftaltitude. Two basic measures of aircraft altitude are available. One isa measure of the distance between the aircraft and the ground. Thismeasurement is commonly made by a radio altimeter or down looking radarwhich measures the transit time of a radio signal generated in theaircraft and reflected from the ground. The other is barometric altitudebased on the pressure of the air through which the aircraft moves.

The radio altimeter is sensitive to minor irregularities in the earth ssurface and to objects such as trees, buildings and the like. If theground were smooth, the radio altimeter rate signal would provide asuitable basis for a ground proximity warning system. However, this isgenerally not the case and the rate signal obtainable in practice causesfalse warnings as a result of the ground surface irregularities. Therate signal derived from the output of a barometric altimeter providesaccurate information regarding the descent or climb of the aircraft, andif the airplane were operating over flat ground, the barometric ratesignal would be satisfactory. Where, however, the ground is not flat,and particularly where it slopes upwardly, the barometric altitude ratesignal alone is not sufficient for reliable operation.

The combination of an amplitude limited radio altimeter rate signal anda barometric altimeter rate signal provides a calculated or syntheticaltitude rate signal which has the ground referenced accuracy of theradio altimeter and the dynamic reliability of the barometric altimeter.A ground proximity warning system utilizing the calculated altitude ratesignal is free of false warnings yet provides ample warning of asituation which requires pilot action.

The novel means for generating an altitude rate signal and otherfeatures of the invention are described as a part of a warning systemwhich is based on the assumption of certain flight conditions andaircraft operating characteristics. This specific warning systemprovides a suitable background for an understanding of the invention.However, the altitude rate signal generator and other features of theinvention could be used in warning or control systems based on otherconditions and characteristics.

FIGS. 1 and 2 illustrate the basis for the warning criteria utilized inthe system disclosed herein. The curve of FIG. 1 shows the trajectory 10of an aircraft 11 during an approach toward the ground 12 and a climbfollowing the occurrence of a warning and the execution ofa pullupmaneuver by the pilot. Assume that the airplane 11 in FIG. 1 is at analtitude, h, of 500 feet and descending at a rate of 40 feet per second.This is a negative rate of change of altitude, h, sometimes referred toas the sink rate of the aircraft. A ground proximity warning is given tothe pilot who waits for 8 seconds T whereupon he initiates a pull-upmaneuver. The pull-up is executed so as to exert an accelerating forceof 0.1 g on the aircraft. 3 is the acceleration due to gravity. Underthese conditions, the aircraft would just touch the ground at the lowpoint of its trajectory.

It would be unusual for a pilot to delay 8 seconds before initiating thepull-up maneuver, following occurrence ofa warning signal. Furthermore,a pull-up of 0.1 g is a mild maneuver, significantly less severe thanthat which a pilot would normally execute in a situation where a warningof ground proximity is received. Accordingly, the low point of thetrajectory of the aircraft which is actually experienced will occur at adistance above the ground which may be expressed as H T k}. T,, is thesum of the pilot reaction delay plus the ground clearance time factor, Tand h is the sink rate at the time of warning.

The conditions for the warning situation illustrated in FIG. l are givenby the trip equation:

1= HT [dun o-n1 where h is the altitude at which the warning occurs andn is the design pull-up factor.

FIG. 2 is a plot of the trip equation curve 15 with the warning altitudeh as the ordinate and the sink rate, h as the abscissa. So long as thealtitude and sink rate of the aircraft place its operation in the zone16 above the curve 15, no warning is given. However, when the operatingconditions of the aircraft are located on or in the zone 17, below thecurve, a warning occurs. The zone below the cure may be considered awarning zone.

The block diagram of FIG. 3 illustrates a system for providing aproximity warning in accordance with the trip equation. An altitudesensor 20 has an output h connected with a differentiator or ratecircuit 21, the output of which is identified as h. Trip equationcomputer 22 continuously calculates the warning altitude h correspondingwith the altitude rate condition of the aircraft. The computer includesa multiplier 23 with the altitude rate signal it connected to both itsinputs. The output of the multiplier is h. This signal is connected withan amplifier 24 having a gain of l/(2ng). The altitude rate signal Ii isconnected with amplifier 25 having a gain T The outputs of the twoamplifiers are combined at summing junction 26, the output of thesumming junction being (h /2ng) HT or h The warning altitude signal h isapplied as a positive input to summing junction 28 while the output, h,of altitude sensor 20, is connected as a negative input to the summingjunction. If the output of summing 28 is positive, it indicates that theaircraft is at an altitude below the warning altitude h. This conditionis detected by comparator 29 which has an output CW, a closure warnsignal. The transfer characteristic of comparator 29 has sufficienthysteresis, indicated by the diagram in the block, to avoid repeatedlyswitching in and out of a warn condition with small variationsin thesignals.

A radio altimeter used as the altitude sensor in the system of FIG. 3would provide a-curate information concerning the distance of theaircraft above the ground, but the system would be subject to excessivefalse warnings when flying over rough terrain. A barometric altimeterwould eliminate the rough ground false warnings but would not warn of adangerous condition resulting from the aircraft traveling over risingground. Furthermore, a barometric altimeter is subject to errors fromincorrect zero setting and long term drift with barometric pressurechanges. The system of FIG. 3, while theoretically accurate, cannotpractically be implemented with either of the commonly used altitudemeasuring instruments.

FIG. 4 shows in block diagram form a complementary filter system forcombining the outputs h of radio altimeter 3t) and k of barometricaltimeter 31. Rate circuits 32 and 33 differentiate the altitude signalsfrom the radio and barometric altimeters 30 and 31, respectively, andhave outputs h and it which are coupled to the inputs of complementaryfilter 34. The radio altitude rate signal h is connected through lowpass filter section 35 and the barometric altitude rate signal 11,, isconnected through high pass filter section 36, the filtered signalsbeing combined at summing junction 37 to provide a calculated altituderate signal li The low pass filter has essentially an integratorcharacteristic while the high pass filter is essentially adifferentiator or washout network. The time constant of complementaryfilter, TC, is selected in accordance with the nature of the terraincharacteristics over which the aircraft operates and the warningsensitivity which is desired. Of course, the more sensitive the warningis made, the greater the likelihood of the occurrence of a false warningwith rough terrain. In practice, a time constant of the order of l to 5seconds has been found satisfactory and, as will appear, the timeconstant may be made variable so that it can be adjusted for differentoperating conditions.

The curve of FIG. 4a illustrates the characteristic of low pass filter35 as a function of the time duration of the signal. FIG. 4b illustratesthe characteristic of high pass filter section 3 6, and FIG. 40 showsthe calculated altitude rate signal h Low pass filter section 35 reducesthe relative effect of high frequency or short term rate changes in theradio altitude rate signal, resulting from ground surfacediscontinuities. The high pass filter section 36 eliminates the longterm barometric altitude rate signal so that offset and drift errors arenot introduced into the system.

It has been found, however, that even with the complementary filter theradio altitude rate signal h often has an amplitude sufficient togenerate a false warning signal. Accordingly, a system based on thecalculated altitude rate signal of FIG. 4 would in practice beunsatisfactory.

FIG. 5 is a block diagram of a preferred form of the warning system. Asin FIG. 4, altitude signals k and h from radio and barometric altimeters40 and 41 provide inputs to the system. In accordance with theinvention, the radio altitude signal h,; is differentiated by ratecircuit 42 and the radio altitude rate signal It}; is connected with arate limiter 43 which prevents the rate signal from exceeding anamplitude limit which may be selected'in'a manner'to bedscr'ibed below.The output of the limiter, Ii is connected with the input of a low passfilter section 44 of complementary filter 45. The barometric altitudesignal 11,, is connected through rate circuit 46 and the barometricaltitude rate signal h,, is connected through switch 47 with the inputof high pass filter section 48. The filtered rate signals are combinedat summing junction 49 and the calculated altitude rate signal liprovides the input to trip equation computer 50.

A multiplier 52 in the trip equation computer has the altitude ratesignal/1} connected with both inputs and having an output h as in FIG.3. This signal is connected with amplifier 53 having a gaincharacteristic of 1/(2ng). The altitude rate signal is also connectedwith amplifier 54 having a gain characteristic T The outputs of the twoamplifiers are combined in the summing junction 55 to provide a signal hthe trip altitude for the altitude rate of the aircraft. An aircraftconfiguration sensor 56 controls the gains of amplifiers 53 and 54 inaccordance with the aircraft pull-up characteristics for eachconfiguration.

The calculated trip altitude is compared with the altitude of theaircraft with respect to the ground, to determine whether a closurewarning should be given. The radio altitude signal 11,; is subtractedfrom the trip altitude signal hat summing junction 57 and the differenceis connected with comparator 58 which provides a closure warning signalwith a positive input. The closure warning signal is connected withaudio warning circuitry to be described below.

The rate limiter 43 eliminates, or at least substantially reduces, thefalse warnings resulting from radio rate signals caused by groundirregularities and the like. The rate limit is preferably set foroperation as a function of the aircraft flight mode. For example, whenthe aircraft is in the final stage of a landing approach, descendingtoward the runway at a low altitude, it is particularly undesirable thata closure warning be given as a result of the radio altimeter sensing aground surface irregularity, a building or the like. Accordingly, theamplitude rate limit is set at a lower level by limiter 43 in thisoperating condition, than when the aircraft is in a cruise mode andpresumably is operating at an altitude where a higher closure rate canbe experienced safely. In the embodiment of the system illustrated inFIG. 5, the rate limit of limiter 43 is selected in accordance with theposition of the aircraft landing gear as determined by landing gearsensor 60. The transfer characteristic of the rate limiter shows twolimiting levels, LGU and LGD, representing conditions of landing gear upand landing gear down, respectively.

The rate levels are preferably determined by the aircraftcharacteristics. In one embodiment of the system, the lower limit is setin accordance with the aircraft recovery characteristics, illustrated inthe curve of FIG. 2. For example, a suitable limit is the sink ratecorresponding with the minimum operating clearance altitude of theaircraft. The upper limit, effective except during l'anding approach,preferably corresponds with the maximum climb characteristics of theaircraft. If an aircraft cannot climb faster than 80 feet per second,for example, there is no need to sense a higher altitude rate.

Comparator 62 and switch 47 provide a cutout of the barometric altitudeinput during take-off and the initial portion of the aircraft climbout.During the take-off run of an airplane, air is compressed in front ofand below the aircraft body and wings. The barometric altimeter sensesan increased pressure and thus indicates a lower altitude. The signal Iiwould indicate that the plane was descending and a false warning wouldoccur. This is at best annoying to the pilot during the take-offmaneuver. Accordingly, comparator 62 senses the radio altitude h andopens switch 47 when the plane is on the ground during the take-off runand during the initial portion of the climb-out. It has been found thatthe ground effect on the barometric altimeter is substantiallydissipated by the time the aircraft reaches a height of 50 feet. Whenthe comparator senses this height, switch 47 closes and remains closeduntil the plane lands. The transfer characteristic of the comparator isillustrated graphically in block 62.

Limiting the negative altitude rate signal avoids the occurrence offalse warnings. However, it is preferable that the positive altituderate signal also be limited to avoid the occurrence of a large signalwhich could charge a capacitor in the complementary filter network andtemporarily block or overload the circuit.

Suitable circuitry for the radio altimeter rate circuit, limiter and thecomplementary filter is illustrated in FIG. 6. The radio altitude signalh R is connected from the altimeter (not shown) to terminals 65, 66, andis coupled through capacitor 67 and series resistor 68 with the negativeinput of operational amplifier 69. Diodes 70, 71, reversely connectedfrom the amplifier input to a reference potential or ground 72, preventoverloading the amplifier with an excessive signal. The positive inputof amplifier 69 is returned through resistor 73 to ground. The output ofoperational amplifier 69 is connected with a bridge limiter 75, biasedto a desired operating level by a voltage divider including resistors76, 77 which connect the bridge between positive and negative voltagesupplies. A feedback network, capacitor 78 and resistor 79, is connectedbetween the output of the bridge and the negative input of amplifier 69,Capacitor 67 provides a differentiating or rate characteristic for theamplifier and the time constant of the feedback network sets thedifferentiating time. A suitable time constant is 0.1 second.

The bias network for bridge limiter 75 is completed through resistor 81,connected from the bridge output to ground. The voltage relationshipestablished in the bridge by the positive and negative voltage suppliesand resistors 76, 77 and 81 establish the level at which the radioaltitude rate signal is limited. When the landing gear of the aircraftis lowered, switch 82 closes connecting resistor 83 in parallel withresistor 81 and decreasing the limiting levels as described above.

Operational amplifier 85 provides the active element of thecomplementary filter. The amplifier has a direct feedback connection 86from the output to the negative input. This operational amplifierconfiguration is sometimes referred to as an unloading amplifier; and ithas the characteristic of substantially an infinite input impedance atthe positive input.

The limited radio rate signal H is connected with the positive amplifierinput through a resistive network including V potentiometer 87 andresistor 88. The barometric altitude rate signal it is connected withthe positive input of amplifier 85 through capacitor 89. The resistiveand capacitive input circuits together with the high impedance unloadingamplifier provide both low pass filtering of the radio rate signal andhigh pass filtering of the barometric rate signal. For example, if thebarometric rate signal is zero, resistors 87, 88 and capacitor 89 act asa simple integrator or low pass filter for the radio rate signal; and ifthe radio ratesignal is zero, capacitor 89 and resistors 88, 87 serve asa high pass, washout or differentiator circuit for the barometric ratesignal. The filtered signals are summed in the amplifier and thecalculated altitude rate signal h is derived from output terminals 93,94 connected with the output of amplifier 85.

The time constant 1- of both sections of the complementary filter may bechanged by varying a potentiometer 87.

Returning again to H6. 5, a circuit is shown for sup plementing thebarometric altitude rate signal with a signal A derived from a normalaccelerometer 102. The barometric altitude signal h is connected withthe high pass filter section 103 of a complementary filter 104. Thenormal accelerometer signal is connected with low pass filter section105. The two filtered signals are combined at summing point 106,providing a dynamic altitude rate signal hp which may be substituted forthe signal h], as an input to the high pass filter section 48 ofcomplementary filter 45. Further details of a dynamic altitude ratecircuit may be found in FIG. 10 of Bateman et a1. application Ser. No.42,918, filed June 3, 1970, assigned to the assignee of thisapplication.

The existence of a closure warning signal at the output of thecomparator 58 actuates a novel audio signal generator which alerts thepilot to the condition of proximity to the ground. Briefly, an audiosignal is modulated at a subaudio repetition rate, and the frequency,repetition rate and amplitude of the signal are selected in accordancewith the relationship of the aircraft to the ground, to indicate to thepilot the nature of this relation.

The closure warning signal CW is connected with an audio logic circuit110. AND circuit 111 has inputs of CW and CW ENABLE, a signal whichindicates the integrity of the inputs to the ground proximity computer.With both signals available the output of AND gate 111 indicates a softwarn condition. This is a first warning criterion. A second warningcriterion is provided by the aircraft altitude. When the aircraft isoperating at an altitude of 500 feet or below, and a soft warningoccurs, an output from AND gate 112 provides a hard warn signal, HW.

In audio generator 115, a frequency generator 116 has an outputconnected with a modulator 117 where it is amplitude modulated by thesawtooth output of a repetition rate generator 118. The modulated audiofrequency is connected with audio amplifier 119, the output of which isconnected to speaker 120 in the cockpit of the aircraft.

With a soft warn condition frequency generator 116 is operated at afrequency f,, and amplifier 119 is set for a gain A,. With a hard warncondition, frequency generator 116 operates a frequency f repetitionrate generator 118 and amplifier 119 at a gain A In a specificembodiment of the system, the frequency f is 400 Hertz. In the hard warncondition, the audio frequency f is doubled to 800 cycles, and theamplitude level A is 15 db higher than A These changes ofcharacteristics of the audio signal afford a clear distinction betweenthe soft warn and the hard warn signal conditions.

D The repetition rate generator is controlled by a signal h obtainedfrom the output of summing junction 57, representing the penetration ofthe aircraft into the undesired operating zone 17 below curve 15 of FIG.2. The degree of penetration of the undesired operating zone is ameasure of the corrective action which should be taken. The signal ii,,is connected with repetition rate generator 118 and varies therepetition rate.

The audio warn system described herein may, of course, be used to alertthe operators of other machines to unsafe or undesirable operatingconditions. Conversely, the signals CW, HW and h, from the groundproximity warning system may be utilized to actuate other types ofwarnings, as lights, for example, to alert the pilot to the groundproximity condition.

Iclaim:

1. An instrument for determining the proximity condition of an aircraftwith respect to the ground, comprising:

means for generating a signal representing the altitude ofthe aircraftwith respect to the ground; means responsive to said altitude signal forgenerating a signal representing the time rate of change of the aircraftaltitude with respect to the ground; means responsive to said altituderate signal, limiting the amplitude thereof;

means for generating an aircraft barometric altitude signal;

means responsive to said barometric altitude signal for generating asignal representing the time rate of change of the aircraft barometricaltitude; means for combining long term components of the limited signalrepresenting the time rate of change of the altitude of the aircraftwith respect to the ground, and short term components of the signalrepresenting the time rate of change of the barometric altitude of theaircraft, to develop a calculated aircraft altitude rate signal; and

means for combining a function of the calculated altitude rate signaland a function of the signal representing the altitude of the aircraftwith respect ground, providing a warning signal when the rate ofapproach of the aircraft toward the ground is excessive for the altitudeof the aircraft with respect to the ground.

2. The instrument of claim 1 in which the amplitude limit ofthe altituderate signal is adjustable.

3. The instrument of claim 2 in which said altitude rate limit isadjusted as a function of the flight mode of the aircraft.

4. The instrument of claim 2 including means sensing a characteristic ofthe aircraft configuration and means for adjusting the limitingamplitude of said aircraft altitude rate signal in accordance with theaircraft configuration.

5. The instrument of claim 1 wherein said altitude rate signal is of onepolarity for a climb and of the other polarity for a descent and saidlimiting means limits signals of each polarity.

6. The instrument of claim 3 having a first altitude rate limit forlanding approach and a second altitude rate limit for other flightmodes.

7. The instrument of claim 6 in which the first limit for landingapproach is selected in accordance with the recovery characteristics ofthe aircraft and the second limit for other flight modes, is selected inaccordance with the maximum climb characteristics of the aircraft.

8. The instrument of claim 1 in which the means for combining thelimited altitude rate signal with the barometric altitude rate signal toprovide the calculated altitude rate signal is a complementary filter.

9. The instrument of claim 8 in which said complementary filter includesa low pass filter section having the limited altitude rate signalconnected therewith, a high pass filter section having the barometricaltitude rate signal connected thereto and means combining the outputsof said filter sections.

10. The instrument of claim 9 in which the transfer characteristic ofsaid low pass filter is l/(r s+l) and in which the transfercharacteristic of the high pass filter is 'm/(T 5+1 11. The instrumentof claim 10 in which the filter time constant, 1, is adjustable.

12. The instrument of claim 11 in which the filter time constant, T, hasa value of the order of l to 5 seconds.

13. The instrument of claim 8 wherein said complementary filtercomprises:

an operational amplifier having a positive input, a

negative input and an output,

a feedback circuit connecting the output of the operational amplifier tothe negative input thereof for operation of the amplifier as anunloading amplifier with a high input impedance,

a resistive circuit connecting said limited altitude rate signal withthe positive input of said amplifier, and

a capacitive circuit connecting said barometric altitude rate signalwith the positive input of said amplifier, the resistive and capacitivecircuits forming a low pass filter for the limited altitude rate signaland a high pass filter for the barometric rate signal.

14. The instrument of claim 1, including:

means for disabling said barometric altitude signal generating meansduring take-off of the aircraft.

15. The instrument of claim 14 including means for maintaining thedisabled condition of said barometric altitude signal generating meansduring the initial portion of a climb-out following take-off.

16. The instrument of claim 14 wherein said means for disabling thebarometric altitude signal generating means includes means responsive tothe signal representing the altitude of the aircraft with respect to theground.

1. An instrument for determining the proximity condition of an aircraftwith respect to the ground, comprising: means for generating a signalrepresenting the altitude of the aircraft with respect to the ground;means responsive to said altitude signal for generating a signalrepresenting the time rate of change of the aircraft altitude withrespect to the ground; means responsive to said altitude rate signal,limiting the amplitude thereof; means for generating an aircraftbarometric altitude signal; means responsive to said barometric altitudesignal for generating a signal representing the time rate of change ofthe aircraft barometric altitude; means for combining long termcomponents of the limited signal representing the time rate of change ofthe altitude of the aircraft with respect to the ground, and short termcomponents of the signal representing the time rate of change of thebarometric altitude of the aircraft, to develop a calculated aircraftaltitude rate signal; and means for combining a function of thecalculated altitude rate signal and a function of the signalrepresenting the altitude of the aircraft with respect ground, providinga warning signal when the rate of approach of the aircraft toward theground is excessive for the altitude of the aircraft with respect to theground.
 1. An instrument for determining the proximity condition of anaircraft with respect to the ground, comprising: means for generating asignal representing the altitude of the aircraft with respect to theground; means responsive to said altitude signal for generating a signalrepresenting the time rate of change of the aircraft altitude withrespect to the ground; means responsive to said altitude rate signal,limiting the amplitude thereof; means for generating an aircraftbarometric altitude signal; means responsive to said barometric altitudesignal for generating a signal representing the time rate of change ofthe aircraft barometric altitude; means for combining long termcomponents of the limited signal representing the time rate of change ofthe altitude of the aircraft with respect to the ground, and short termcomponents of the signal representing the time rate of change of thebarometric altitude of the aircraft, to develop a calculated aircraftaltitude rate signal; and means for combining a function of thecalculated altitude rate signal and a function of the signalrepresenting the altitude of the aircraft with respect ground, providinga warning signal when the rate of approach of the aircraft toward theground is excessive for the altitude of the aircraft with respect to theground.
 2. The instrument of claim 1 in which the amplitude limit of thealtitude rate signal is adjustable.
 3. The instrument of claim 2 inwhich said altitude rate limit is adjusted as a function of the flightmode of the aircraft.
 4. The instrument of claim 2 including meanssensing a characteristic of the aircraft configuration and means foradjusting the limiting amplitude of said aircraft altitude rate signalin accordance with the aircraft configuration.
 5. The instrument ofclaim 1 wherein said altitude rate signal is of one polarity for a climband of the other polarity for a descent and said limiting means limitssignals of each polarity.
 6. The instrument of claim 3 having a firstaltitude rate limit for landing approach and a second altitude ratelimit for other flight modes.
 7. The instrument of claim 6 in which thefirst limit for landing approach is selected in accordance with therecovery characteristics of the aircraft and the second limit for otherflight modes, is selected in accordance with the maximum climbcharacteristics of the aircraft.
 8. The instrument of claim 1 in whichthe means for combining the limited altitude rate signal with thebarometric altitude rate signal to provide the calculated altitude ratesignal is a complementary filter.
 9. The instrument of claim 8 in whichsaid complementary filter includes a low pass filter section having thelimited altitude rate signal connected therewith, a high pass filtersection having the barometric altitude rate signal connected thereto andmeans combining the outputs of said filter sections.
 10. The instrumentof claim 9 in which the transfer characteristic of said low pass filteris 1/( Tau c s+1) and in which the transfer characteristic of the highpass filter is Tau cs/( Tau c s+1).
 11. The instrument Of claim 10 inwhich the filter time constant, Tau , is adjustable.
 12. The instrumentof claim 11 in which the filter time constant, Tau , has a value of theorder of 1 to 5 seconds.
 13. The instrument of claim 8 wherein saidcomplementary filter comprises: an operational amplifier having apositive input, a negative input and an output, a feedback circuitconnecting the output of the operational amplifier to the negative inputthereof for operation of the amplifier as an unloading amplifier with ahigh input impedance, a resistive circuit connecting said limitedaltitude rate signal with the positive input of said amplifier, and acapacitive circuit connecting said barometric altitude rate signal withthe positive input of said amplifier, the resistive and capacitivecircuits forming a low pass filter for the limited altitude rate signaland a high pass filter for the barometric rate signal.
 14. Theinstrument of claim 1, including: means for disabling said barometricaltitude signal generating means during take-off of the aircraft. 15.The instrument of claim 14 including means for maintaining the disabledcondition of said barometric altitude signal generating means during theinitial portion of a climb-out following take-off.
 16. The instrument ofclaim 14 wherein said means for disabling the barometric altitude signalgenerating means includes means responsive to the signal representingthe altitude of the aircraft with respect to the ground.